Reactivation of NBD-phallacidin-labelled actomyosin fibrils in cryosections of Physarum polycephalum: a new cell-free model.

Application of ATP to cryosections of plasmodial strands from Physarum polycephalum leads to an isotonic contraction of the cytoplasmic actomyosin fibrils: when the fibrils are labelled with NBD-phallacidin, their contraction can be observed in the fluorescence microscope. While performing contraction, the fibrils separate into many small units and the formerly continuous fibrils exhibit the appearance of beaded chains. The possibility of visualizing directly the contraction of cytoplasmic actomyosin fibrils in the fluorescence microscope represents a favourable condition for the study of their physiological contraction mechanism, because this new and convenient cell-free model offers in situ contractile structures that are non-denatured and non-extracted.